New applications require nanofibers produced from a variety of materials including polymer melts, nanoparticles such as carbon nanotubes and liquid solutions. The diameters of such fine fibers can range in size from submicron to several microns depending on the functional requirements. There is also increased demand for loading various drugs and other active ingredients into fine fibers for the production of topical or systemic wound dressings, sublingual or oral drug delivery systems. There are several methods for producing small diameter fibers using high-volume production methods, such as flash-spinning, island-in-sea, and melt-blowing. However, the usefulness of the above methods is restricted by combinations of narrow material ranges, high costs and difficulty in producing submicron diameter fibers.
Electrospinning is a simple and well established process for producing fine fibers from solutions. Electrospinning is a process for submicron scale polymer-based filament production by means of an electrostatic field and solvent evaporation. The principal limitations of electrospinning are a very low productivity and the use of organic solvents which are difficult and costly to fully remove. Electrospinning is not well suited to produce fine fibers from fiber forming materials such as polymer melts as the much higher viscosity requires greater electrical fields leading to arcing. Electrospinning of solutions with high loading of particles is also very difficult. Drug loading is always a problem and loads greater than 5% are difficult to achieve with electrospinning. Furthermore, high drug/particle loading will often result the uneven distribution of the drug/particle in the electrospun fine fiber matrix resulting in initial burst effects. See Electrospun nanofibers-based drug delivery systems. D G Yu et al. Health I (2009). Despite the versatility and popularity of electrospinning, high-voltage electrical fields, sensitivity to variability in solution conductivity, low production rate, solvent based processing and difficulty in drug loading limit its application.
Rotary-jet spinning is another method in the early development stage which seeks to overcome some of the above listed limitations of electrospinning of nanofibers. U.S. Pat. No. 7,134,857 to Andrady et al. The method uses a high-speed rotating nozzle to form a polymer jet which undergoes extensive stretching before solidification. The system consists of a reservoir containing a polymer in solution with two side wall orifices that was attached to the shaft of a motor with controllable rotation speed. The outward radial centrifugal force stretches the polymer jet as it is projected toward a collector wall, but the jet travels in a diffuse trajectory due to rotation-dependent inertia. Concurrently, the solvent in the polymer solution evaporates, solidifying and contracting the jet.
Another problem with the processing of polymers into fine fibers is that such processes generally involve organic solvents which can be highly toxic and damaging to the environment. Flash-spinning, jet-spinning, electrospinning and electro-blown spinning typically require that the polymer be dissolved in a solvent. While manufacturing processes typically involve the removal of organic solvents, such processes require specially equipped manufacturing facilities. Additionally, small quantities of organic solvents still remain and may leach from the fibers over time. Such solvent residues can be problematic in sensitive biological applications. The limited availability of ecologically friendly manufacturing processes has been a major barrier to the greater use of biodegradable polymers in the medical field. There is therefore a need for a production process capable of producing fine fibers of controllable diameter size and distribution without the use of organic solvents.
Several research efforts have involved the formation of fine fibers directly from melts. One important advantage of creating fine fibers from polymer melts is that the dissolution of polymers in organic solvents and the subsequent removal/recycling of solvents are no longer required. Meltblowing processes manage the separate flow of process gases, such as air, and polymeric material through a die body to effect the formation of the polymeric material into continuous or discontinuous fiber. In most known configurations of meltblowing nozzles, hot air is provided through a passageway formed on each side of a die tip. The hot air heats the die and thus prevents the die from freezing as the molten polymer exits and cools. In this way the die is prevented from becoming clogged with solidifying polymer. In addition to heating the die body, the hot air, which is sometimes referred to as primary air, acts to draw, or attenuate the melt into elongated micro-sized filaments. In some cases, a secondary air source is further employed that impinges upon the drawn filaments so as to fragment and cool the filaments prior to being deposited on a collection surface. Meltblown fibers are known to consist of fiber diameters of 1 to 10 microns. Further reduction of meltblown fiber size to submicron ranges is typically difficult, requiring a combination of smaller capillary size, lower polymer throughput per capillary, increased number of capillaries per die width to compensate for the lower throughput, specialized polymer rheology, and control of polymer cooling temperature as filaments solidify. (See Melt blown nanofibers: Fiber diameter distributions and onset of fiber breakup, Christopher J. Ellison, Alhad Phatak, David W. Giles, Christopher W Macosko, Frank S. Bates, Polymer 48 (2007) 3306-3316)
U.S. Pat. Nos. 5,260,003 and 5,114,631 to Nyssen, et al., both hereby incorporated by reference, describe a meltblowing process and device for manufacturing ultra-fine fibers and ultra-fine fiber mats from polymers with mean fiber diameters of 0.2-15 microns. Laval nozzles are utilized to accelerate the process gas to supersonic speed; however, the process as disclosed has been realized to be prohibitively expensive both in operating and equipment costs. U.S. Pat. No. 6,800,226 to Gerking, hereby incorporated by reference, teaches a method and a device for the production of essentially continuous fine threads made of meltable polymers. The polymer melt is spun from at least one spin hole and the spun thread is attenuated using gas flows which are accelerated to achieve high speeds by means of a Laval nozzle. The air is rapidly accelerated as it passes the converging section of the nozzle. The polymer melt is attenuated by the air jet until the fiber bursts open and disintegrates into a multitude of finer filaments. Nonwoven fabrics made of fibers with diameters from 2 to 5 microns have been successfully fabricated using this process.
More recently, methods of forming fibers with fiber diameters less than 1.0 micron, or 1000 nanometers, have been developed. These fibers are often referred to as ultra-fine fibers, sub-micron fibers, or nanofibers. Methods of producing nanofibers are known in the art and often make use of a plurality of multi-fluid nozzles, whereby an air source is supplied to an inner fluid passageway and a molten polymeric material is supplied to an outer annular passageway concentrically positioned about the inner passageway. One such process, referred to as melt-film fibrillation includes the steps of utilizing a central fluid stream to form an elongated hollow polymeric film tube and using high velocity air to shear multiple nanofibers from the hollow tube.
U.S. Pat. No. 6,382,526 and U.S. Pat. No. 6,520,425 to Reneker, et al., both hereby incorporated by reference, disclose such a melt film fibrillation process for producing nanofibers. Fiber forming material is forced concentrically into a thin annular film around an inner concentric passageway of pressurized gas. This film is subjected to shearing deformation by an outer concentric gas jet until it reaches the fiber-forming material supply tube outlet. At this point, expansion of this inner pressurized gas stream is said to eject the “fiber-forming material from the exit orifice of the annular column in the form of a plurality of strands of fiber-forming material that solidify and form nanofibers having a diameter up to about 3,000 nanometers.
U.S. Pat. No. 4,536,361 to Torobin, incorporated herein by reference, teaches a similar microfiber formation method wherein a coaxial blowing nozzle has an inner passageway to convey a blowing gas at a positive pressure to the inner surface of a liquid film material, and an outer passageway to convey the film material. The combined action of the expansion of the blowing gas and an entraining fluid jet impinging at a transverse angle fracture the film to form microfibers. Drawbacks of the film fibrillation processes are that they require multiple pressurized gas streams which complicate nozzle design and they do not readily produce fibers smaller than meltblown fibers. There is therefore a need for a production process capable of producing submicron fibers of controllable diameter size and distribution from polymer melts.
There is also a need for producing fine fiber webs of high uniformity and loft.
Additionally, there is a need for a high-throughput process capable of producing large numbers of fine fibers per spinning nozzle.